Two types of polytetrafluoroethylene resins are available commercially, viz, granular resin and fine powder resin. Granular resin is made by polymerizing tetrafluoroethylene in an aqueous medium under conditions which cause the polymer to coagulate during the polymerization reaction to form particles which generally exceed 500 microns in diameter. The resin is then comminuted to smaller particle sizes, e.g., 30 to 100 microns, for molding by such techniques as preforming and sintering or for ram extrusion.
The fine powder resin is made by polymerizing tetrafluoroethylene in an aqueous medium under conditions which maintain the polymer dispersed as fine particles 0.05 to 0.5 micron in size in the medium until the polymerization reaction is completed. The particles in the aqueous dispersion can then be coagulated and dried, and are useful in this form for paste extrusion.
Two main differences between the processes for making these resins is that (a) stirring in the granular polymerization system is more vigorous than in the fine powder polymerization system, causing coagulation during the polymerization reaction and (b) sufficient dispersing agent is present in the fine powder polymerization system to maintain the polymer particles dispersed until the polymerization reaction is completed, whereas the amount, if any, of dispersing agent present in the granular polymerization system is insufficient to give this result.
Commercially available fine powder resins are not normally fabricable by commercial molding and ram extrusion processes used for granular resin; and granular resin is not fabricable by the paste extrusion techniques by which fine powder resin is most commonly processed.
The present invention arises in the field of fine powder resins and their aqueous dispersion precursors. In the paste extrusion of these fine powder resins, the resin is blended with lubricant to form lubricated agglomerates which are precompacted and charged to an extruder barrel and extruded at about room temperature through a die with a cross-section much smaller than that of the barrel. The resulting extrudate is then heated to remove lubricant and usually sintered by heating to coalesce the residual resin into an integral mass. A common commercial fine powder paste extrusion application is extrusion onto a wire to insulate the wire. An adverse property of these fine powder resins is their tendency to develop shear faults or flaws when extruded as a coating onto the wire at high reduction ratios. (Reduction ratio is the ratio of the cross-sectional area of the extruder barrel to the cross-sectional area of the extruder die.) It has been found that each distinct fine powder resin has a certain maximum reduction ratio above which the resin tends to develop flaws as it is extruded. At higher ratios, the resin may actually shatter as it is sintered after it is extruded onto the wire. The reason for the appearance of flaws in the coating as the reduction ratio is increased is not entirely understood but is believed to be due to the shear stresses built up at the entrance to the die as the reduction ratio is increased. Resins capable of extrusion at high reduction ratios are desirable because the higher the ratio, the larger the barrel that can be used, permitting extrusion of longer continuous lengths of coated wire without reloading the barrel. Thus, the search for fine powder resins capable of extrusion onto wire at high reduction ratios and exhibiting few or no flaws after sintering is a continuing one.
The search is complicated by two facts. Firstly, many prior art reports of fine powder resins capable of being extruded at high reduction ratios are based on visual examination for flaws occurring during extrusion of beading --i.e., solid cylindrical extrudate -- instead of on sintered wire coating. The former is a less sensitive test because flaws in the beading are detected by visual inspection, while flaws in wire coated for electrical use are found by electrically testing in order to detect much smaller flaws (whose detection is of importance in electrical applications). Thus, reports of prior art fine powder resins that can be used at high reduction ratios are frequently misleading because they are based on gross visual inspections. Secondly, fine powder resin coatings, as extruded on the wire, are unsintered and upon sintering the coated wire for enduse applications, additional flaws appear in the coating. But much of the past work, as evidenced by the prior art in this area, has not considered the flaws that appear during sintering and again has erroneously reported fine powder resins of good extrusion quality having high reduction ratios. In reality, however, because of the flaws appearing during sintering, useful reduction ratios of such resins produced by such past work are much lower.
In summary, in the past, the quality of a fine powder resin for paste extrusion has been measured by paste extruding unsintered beading and visually examining the beading for flaws. As a result, resins have been reported acceptable for extrusion onto wire at reduction ratios as high as 10,000:1. However, though the correlation of unsintered beading extrudate having few flaws and sintered wire coating extrudate having few flaws may be valid when resins are paste extruded at low reduction ratio (e.g., 1950:1 or less), the correlation breaks down when extrusion is carried out at higher reduction ratios. In other words, resins which the art has said are extrudable to produce acceptable unsintered beading at reduction ratios of over 1950, are, in reality, unacceptable to produce sintered wire coatings at the reduction ratios said to be acceptable. For example, Cardinal et al., U.S. Pat. No. 3,142,665, discloses fine powder resins that are said to produce acceptable unsintered beadings at reduction ratios of 10,000:1 or more; however, as shown in the Comparisons hereinbelow, resin produced according to the Cardinal et al. patent has numerous flaws when extruded and sintered on wire at a reduction ratio of only 1930:1. On the other hand, the Examples hereinbelow, e.g., Example 1, show that resins of this invention had few flaws when extruded and sintered on wire at a reduction ratio of 1930:1 and 2840:1.